50 AERATION AND AIR-CONTENT. 



average of 7 parts of COo per million in rain-water and streams, and 

 of 3 parts in shallow ponds, while a lake ranged from 2 parts at 1 foot 

 to 17 at 50 feet. Birge and Juday found 5 to 9 c.c. per Hter near the 

 bottom in the winter, the water above 10 feet being practically 

 without acid during the summer. Bergmann showed that bog- 

 water contained several times as much carbon dioxid as lake-water, 

 the respective averages being 7.8 c.c. and 1.15 c.c. 



INFLUENCE OF ALG^ AND WATER PLANTS ON OXYGEN-CONTENT. 



Brizi (1906 : 89) was the first to point out the significance of 

 algae in the aeration of higher plants. He demonstrated that the 

 alg2e from the rice-fields, when placed in cultures of rice containing 

 CO2 but no oxygen, produced sufficient oxygen to aerate the roots 

 and insure healthy growth. He concluded from this that the algse 

 of the rice-fields greatly increased the amount of oxygen in the water 

 and were the principal factor in the aeration of the roots. They 

 were regarded as of further advantage in consuming the carbon 

 dioxid that might otherwise accumulate in injurious quantities. 

 Chambers (1912 : 203) found that the photosynthesis of rapidly 

 growing algae and aquatic plants in a body of water may diminish 

 or deplete the supply of carbon dioxid and increase the oxygen-content 

 beyond saturation. In the absence of free CO2, the plants may use 

 the half-bound carbon dioxid of the dissolved carbonates, chiefly 

 those of calcium and magnesium. Photosynthesis may be so active 

 as to use up the half-bound carbon dioxid and make the water alka- 

 line, but more carbonates may then be formed as a result of respira- 

 tion and absorption from the air. Waters rich in calcium carbon- 

 ates are also rich in vegetation, but bog-waters containing humic 

 acids, and hence poor in carbonates of lime, are poor in vegetation. 



Harrison and Aiyer (1913 : 94) were led to the conclusion that the 

 surface film of algae on rice soils is the chief agency in the aeration 

 of the roots, as indicated by the evolution of oxygen from such 

 soils. In a special study of this film (1914), they determined that 

 green algae were generally, and diatoms invariably, present, and 

 showed that it decomposed CO2 in the sunlight, with evolution of 

 oxygen. The film probably also contains bacteria capable of oxi- 

 dizing methane and hydrogen to carbon dioxid and water, as it 

 carries on these processes actively. The organic film on the surface 

 of the swampy soil of rice-fields utilizes the gases in such a way as to 

 increase the evolution of oxygen and the consequent aeration. The 

 bacterial activity results in the production of more carbon dioxid, 

 and this is used by the algae with increased production of oxygen. 

 The film permits the maximum oxygen concentration of the water 

 entering the soil. Green manuring increases the soil-gases, supplies 

 more material to the film, and thus augments the evolution of oxygen 

 and consequent aeration. 



